This invention relates to inductive line harmonic suppression in power converters with AC Input, and more particularly to a capacitive boost circuit that allows the use of smaller inductors by providing an earlier onset of conduction during positive and negative half-cycles of the AC supply.
Regulations EN 61000 and IEC 1000 require line harmonic current control in Electrical equipments. The typical way of complying with this requirement is to add an inductor in the input line to enlarge the conduction angle of the AC line current. The amount of line harmonic current (LHC) suppression depends on the inductance value. The major disadvantage is that, firstly, the inductor copper loss is high. Secondly, the input capacitor is not fully charged up to the input peak voltage because of the delay of the charge current when the corner frequency formed by the inductor and the bulk capacitor is below the line input frequency. This means that a bigger bulk capacitor is required to meet the typical hold-up time requirement.
Conventional active power factor control (PFC) circuits can reduce the LHC effectively, but both the part count and cost are high. A simple version of single stage PFC solution is a simple and good solution, but it is only practical for a lower power application. It is limited to a wide-range design, and an expensive bulk capacitor is required to provide the required hold-up time. The dual input range PFC requires an additional power switch and a complicated coupled power transformer. And thus it is also more expensive.
It is therefore desirable to find a way to reduce the size of the harmonic suppression inductors and the bulk capacitors in an AC-to-DC power converter without adversely affecting the LHC suppression capabilities of the circuit.
The present invention enhances the harmonic suppression action of the AC line inductor by providing a capacitive boost circuit which stores energy on the negative half cycle of the AC supply and subsequently releases it at the beginning of the positive half cycle so as to initiate the current flow through the line inductor earlier than the unaided bulk capacitor(s) would permit.
Broadly stated, the present invention comprises a line harmonic control circuit for a power supply circuit having an AC input across a line and a neutral, a bridge rectifier connected to said AC input for full-wave rectifying said AC input, an inductor connected between said AC input and said bridge rectifier, and a bulk capacitance connected to the output of said bridge rectifier for providing a substantially level DC output across a positive terminal and a negative terminal, comprising: a DC voltage source, a first diode and a second diode connected in series between said positive and negative terminals; and a boost capacitor connected between the junction of said first and second diodes and said line, wherein said first and second diodes are of a polarity to allow current flow from said voltage source toward said positive terminal, and wherein the positive side of said voltage source is the side connected to said first diode.